Upgrade licences from 5.0 to 6.0 but keep some old 5.0 and use 6.0 for new hosts

Hi,

I have Vcenter 5 installed and ESX5 hosts and I have also installed a second datacenter on a temporary license a vscenter 6 and ESX6 hosts.

I can upgrade my licenses to version 6.0, but I want to keep my old hosts on 5.0, mainly because they are live and haven't done an upgrade before.

Can I keep my old hosts on 5.0?

Will I need to deactivate the old licenses to do that which means I will need to upgrade all the hosts to 6.0 anyway?

Can I do a partial upgrade?

Or do I need new v6.0 licenses?

I have  Windows vCenter Server 5 Standard on vCenter version 5.0

some hosts on VMware vSphere 5 Foundations license

some hosts on VMware vSphere 5 Standard License 

Thanks